The present invention relates to subterranean treatment operations, and more particularly, to environmentally friendly, low-temperature filter cake clean-up fluids and related methods.
A subterranean treatment fluid used in connection with a subterranean formation may be any number of fluids (gaseous or liquid) or mixtures of fluids and solids (e.g., solid suspensions, mixtures and emulsions of liquids, gases and solids) used in subterranean operations. An example of a subterranean treatment fluid is a drilling fluid. Drilling fluids are used, inter alia, during subterranean well-drilling operations to, e.g., cool the drill bit, lubricate the rotating drill pipe to prevent it from sticking to the walls of the well bore, prevent blowouts by serving as a hydrostatic head to counteract the sudden entrance into the well bore of high pressure formation fluids, and also remove drill cuttings from the well bore. Another example of a subterranean treatment fluid is a “drill-in and servicing fluid,” which includes fluids placed in a subterranean formation from which production has been, is being, or may be cultivated. For example, an operator may begin drilling a subterranean borehole using a drilling fluid, cease drilling at a depth just above that of a potentially productive formation, circulate a sufficient quantity of a drill-in and servicing fluid through the bore hole to completely flush out the drilling fluid, then proceed to drill into the desired formation using the well drill-in and servicing fluid. Drill-in and servicing fluids often are utilized, inter alia, to minimize damage to the permeability of such formations.
Subterranean treatment fluids generally are aqueous-based or oil-based, and may comprise additives such as viscosifiers (e.g., xanthan) and fluid loss control additives (e.g., starches). Subterranean treatment fluids further may comprise bridging agents, which may aid in preventing or reducing loss of the treatment fluid to, inter alia, natural fractures within the subterranean formation. Calcium carbonate is an example of a conventional bridging agent. In certain circumstances, a bridging agent may be designed to form a filter cake so as to plug off a “thief zone” (a portion of a subterranean formation, most commonly encountered during drilling operations, into which a drilling fluid may be lost). Generally, bridging agents are designed to form fast and efficient filter cakes on the walls of the well bores within the producing formations to minimize potential leak-off and damage. Generally, the filter cakes are removed before hydrocarbons are produced from the formation.
Filter cakes are complex structures involving a cake structure formed from solids present in the drilling or drill-in fluids used to form the well bore and polymeric materials that are present in such fluids. Filter cakes are impermeable structures (as can be differentiated from the porous structure of the subterranean rock). Because of this impermeability, it can be difficult to develop effective methodologies to remove the filter cake for production due to the differing equations of state and solubilization parameters that the filter cake has. The kinetics of fluid flow and dissolution of the filter cake are also different from the surrounding rock because they are affected by the polymer and any other components that may be present in the filter cake.
Conventionally, prior to placing the well under production, filter cakes have been removed from well bore walls by contacting the filter cake with one or more subsequent fluids. For example, where an aqueous-based treatment fluid comprising bridging agents is used to establish a filter cake, operators conventionally have employed enzymes and oxidizers to remove the viscosifier and fluid loss control additive, and then used an acid (or an acid precursor that produces an acid after a delay period) to clean up the calcium carbonate bridging agent. The purpose of the acid is to dissolve the acid-soluble materials in the filter cake (e.g., calcium carbonate); the purpose of the oxidizers and enzymes is to degrade the polymer within the filter cake deposited by various polymeric agents used in the drilling or drill-in fluids.
Acid-based removal methods can be problematic, however, because the strong acid often corrodes metallic surfaces of completion equipment (e.g., sand control screens), thereby causing such equipment to potentially prematurely fail. Further, the strong acid may damage the producing formation. Additionally, the strong acid may cause the bridging agent to dissolve prematurely in a specific area of the well bore interval, resulting in the loss of the strong acid into the formation, before it can completely cover the entire well bore interval filter cake. This is especially problematic in wells involving long intervals or deviated well bore geometries.
Acid-based systems also may be difficult to control. At higher temperatures, the hydrolysis reaction is expedited, and the challenge becomes how to slow the reaction rate to cover the entire interval and remove all of the filter cake present therein because the acid will react too rapidly at the well bore. Corrosion problems are also more aggravated at higher temperatures. In addition, some clay minerals are sensitive to low pH and the introduction of a low pH fluid could cause damage to the formation.
As an alternative to acid-based systems some chelating agents have been used such as EDTA, HEDTA, and NTA. However, these chelating agents can have solubility problems in certain brines, and temperature limitations for use.